Intrinsic Characteristics Of Gasification Slags, And The Phase Evolution Of Gasification Slags During Carbothermal Reduction-nitridation Process ~*

In this paper, gasification slags produced by several kinds of gasifier were selected for the study. Firstly, X-ray fluorescence spectrometer(XRF), X-ray powder diffractometry(XRD), scanning electron microscopy(SEM) and other research methods were used to investigate the intrinsic characteristics of gasification slags. The evolution of phase composition of slags and changes of viscosity at different temperatures were explored through high-temperature melting, quenching process and Fact Sage software simulation method. Then Sialon powders were synthesized by carbothermal reduction-nitridation using gasification slags as main raw materials, graphite as the reductant and white dextrin as binder. In order to investigate the impact of nitridation temperature and holding time on phase composition and microstructure of the final samples, carbothermal reduction-nitridation of clay were introduced for comparison. Thus, the evolution of phase composition and microstructure of gasification slag and clay during carbothermal reduction-nitridation process were revealed. The result showed:(1) The main chemical constituents of gasification slags are Si O2, Al2O3, Ca O and Fe2O3. Due to the short residence time of the coal in the furnace, there is still a large amount of residual carbon in the slags. The main phases in gasification slag were amorphous glass and carbon, which is related to its forming process. In addition, there is a small amount of quartz and calcite phase. By observing the microstructure of gasification slags, a number of spherical glass particles with smooth surface and porous carbon could be found.(2) Chemical composition and phase composition at high temperature determines the variation of viscosity of slags. Ca O-Al2O3-Si O2 ternary phase diagram drawn by Phase Diagram module of software Fact Sage can also be well used to explain the high-temperature phase variation of gasification slags. As can be seen from the experimental results, the ash melting temperature of gasification slags varied with its acid/alkali ratio p. The larger the ratio p was, the higher the melting temperature was. As the temperature increased, the melt viscosities of gasification slags at high temperature were decreased gradually. When the acid/alkali ratio p and temperature was over 2.5 and 1200℃, respectively, the melt viscosities of slags were decreased dramatically.(3) Carbothermal reduction–nitridation results of gasification slags showed that: the major phases of nitridation products at lower temperatures(<1400℃) were calcium feldspar(Ca Al2Si2O8) and O’-Sialon. As the temperature increased, main phase of nitridation products was transformed to Ca-α-Sialon. Other impurities such as Al N and Si C phase were also formed in high temperature. The results of SEM showed that the spherical morphology with regular shape in the raw material was retained in samples nitridated at a relatively low temperature. As the temperature increased, the spherical surface became rough, and even breakage. At high temperatures, the surface was covered with short columnar granular material. Furthermore, with the penetration of nitrogen, a hollow shape was formed inside the sphere. Therefore, in a way, the extent of deformation and breakage in the sphere may reflect the nitridation degree of gasification slags. With the rising of the temperature or extension of holding time, Ca-α-Sialon phase in the nitride products of the gasification slags gradually generated and developed fully. When the temperature was higher than 1400℃ and the holding time was 4h, intertwined columnar Ca-α-Sialon of the nitridation products could be clearly observed. When the temperature continued to rise to 1450℃, Sialon phase becomed stabilized, and the impurity phase was the least.